Here, on ReefPedia you can find quite a few articles on pH in a marine tank. For example, from the article The role of pH in a marine aquarium, you will learn the basics of how pH affects the living of saltwater organisms.

In the article you’re currently reading, I’ll briefly take a closer look at the fluctuation of the pH value in a marine aquarium over the day and night.

Daily pH fluctuations

The diurnal change of pH in marine aquaria is caused by the biological processes of photosynthesis and respiration of the animals. During photosynthesis, organisms convert carbon dioxide into oxygen and energy (using light).

This can be expressed by the following equation:

6CO2 + 6H2O + light => C6H12O6 (carbohydrates) + 6O2

This process leads to the consumption of carbon dioxide. Consumption in turn leads to lowering of CO2 during the day and an increase in pH. The result of these processes is an increase in pH during the day and a decrease at night.

A good way to stabilise diurnal fluctuations is to grow a refugium and illuminate it at night. This will lead to an equalisation of the day-night pH differences.

Higher carbonate alkalinity leads to lower pH fluctuations, as the combination of carbonate and bicarbonate buffer prevents pH changes.

Both methods are more effective at a pH of 8.5 than at a low pH of 7.8, so aquaria with a lower pH can see greater fluctuations.

Summary

I have written a whole series of articles on the importance of pH in marine aquariums – you can find them all on ReefPedia. I encourage you to read them; it is a complete resource to give you a comprehensive understanding of pH in marine aquariums.